Sensor and method of fabrication

ABSTRACT

A sensor ( 10 ) includes a cavity ( 31 ) formed by a substrate ( 11 ), an adhesive ( 21 ), and a filter ( 22 ). A sensing element ( 14 ) is located inside the cavity ( 31 ) while electrical contacts ( 17, 18 ) coupled to the sensing element ( 14 ) are located outside the cavity ( 31 ). The filter ( 22 ) protects the sensing element ( 14 ) from physical damage and contamination during die singulation and other assembly processes. The filter ( 22 ) also improves the chemical sensitivity, selectivity, response times, and refresh times of the sensing element ( 14 ).

The present application is a divisional on prior U.S. application Ser.No. 08/620,729 filed on Mar. 25, 1996, now U.S. Pat. No. 5,798,556,which is hereby incorporated by reference, and priority thereto forcommon subject matter is hereby claimed.

BACKGROUND OF THE INVENTION

This invention relates, in general, to semiconductor devices, and moreparticularly, to sensors.

The packaging process for sensors is labor intensive, time consuming,and expensive. For chemical sensors, the packaging process includessawing a semiconductor substrate into individual chemical sensor chips.Then, the individual chemical sensor chips are separately bonded to andassembled in a bulky metal package known in the art as a T39 package ora T05 package. An example of a T05 package is described in U.S. Pat. No.4,768,070, issued to Takizawa et al. on Aug. 30, 1988. This piece-partpackaging process is slow and tedious and requires careful handling ofthe individual chemical sensor chips, which may become contaminated andphysically damaged during the packaging process.

Accordingly, a need exists for a sensor that is packaged using a batchprocessing technique that improves throughput and reduces cycle time forfabricating and packaging a sensor. The wafer-level batch packagingtechnique should produce a packaged sensor that is compact in size andshould also protect each sensor chip from contamination and physicaldamage during subsequent handling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of an embodiment of a sensorin accordance with the present invention; and

FIG. 2 portrays a partial cross-sectional view of an alternativeembodiment of the sensor of FIG. 1 in accordance with the presentinvention; and

FIG. 3 depicts a partial cross-sectional view of another alternativeembodiment of the sensor of FIG. 1 in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning to the figures for a more detailed description, FIG. 1illustrates a cross-sectional view of an embodiment of a sensor 10.Sensor 10 is a semiconductor component that includes a substrate 11.Substrate 11 has a surface 19 opposite a surface 20 and is typicallycomprised of a semiconductor material such as, for example, silicon, aIII-V compound semiconductor, or a II-VI compound semiconductor.

It is understood that a plurality of sensors can be fabricated onsubstrate 11. For example, FIG. 1 depicts portions of sensors 34 and 35on substrate 11 and adjacent to sensor 10. FIG. 1 also portrays lines 36and 37, which serve as scribe lines for singulating sensor 10 apart fromsensors 34 and 35, respectively.

An electrically insulating layer 32 is provided over surface 19 ofsubstrate 11. Electrically insulating layer 32 is preferably adielectric material such as, for example, silicon oxide or siliconnitride and can be deposited overlying substrate 11 using techniquesknown in the art.

Substrate 11 has an optional recess 12 formed in a portion of surface 20to facilitate heat dissipation in sensor 10 as described hereinafter.Recess 12 extends from surface 20 toward surface 19 and can expose aportion of electrically insulating layer 32. To ensure a manufacturableprocess for sensor 10, recess 12 is preferably etched into surface 20using an anisotropic etchant that etches along specific crystal planesof substrate 11. The anisotropic etchant should not significantly etchelectrically insulating layer 32 compared to substrate 11. Examples ofanisotropic etchants that are suitable for use with single crystalsilicon substrates include, but are not limited to, potassium hydroxide,ammonium hydroxide, cesium hydroxide, hydrazine,ethylenediamine/pyrocatechol, and tetramethylammonium hydroxide.

Sensor 10 also includes a sensing element 14, which is supported byelectrically insulating layer 32 and substrate 11 and which overliesrecess 12. When sensor 10 is a chemical sensor, sensing element 14 istypically a resistor whose resistance changes upon exposure to aspecific liquid or gas (not shown). At elevated operating temperatures,the resistivity of sensing element 14 is typically about 1 kiloohm-50megaohms. As known in the art of chemical sensors, the presence of aspecific liquid or gas is transformed from a chemical reaction into anelectrical signal by a sensor. As an example, a control circuit (notshown) can detect a change in the resistivity of sensing element 14 bymeasuring a change in a current or voltage drop across sensing element14. The control circuitry can be located on a different substrate or canbe fabricated in substrate 11 to create an integrated chemical sensorsystem.

Sensing element 14 is provided or formed over electrically insulatinglayer 32 and surface 19 of substrate 11 using techniques known in theart. When sensor 10 is a chemical sensor, sensing element 14 iscomprised of an electrically conductive and chemically sensitive filmincluding, but not limited to, metal oxides, transition metals, or noblemetals. For example, sensing element 14 can be comprised of tin oxide,zinc oxide, titanium oxide, or an alloy of platinum and gold. Differentcompositions of sensing element 14 permit the sensing or monitoring ofdifferent liquids or gases. It is understood that the material used forsensing element 14 can be doped to further improve the chemicalsensitivity and selectivity of sensing element 14 and sensor 10.

Sensing element 14 can be heated by an optional heating element 13 tohelp catalyze a chemical reaction between sensing element 14 and thedesired liquid or gas. Heating element 13 is formed using techniquesknown to those skilled in the art. As an example, heating element 13 canbe comprised of polysilicon or a metal such as platinum, gold, or thelike.

As illustrated in FIG. 1, heating element 13 is located withinelectrically insulating layer 32, overlies recess 12, and underliessensing element 14. It is understood that heating element 13 can belocated on a different substrate than substrate 11. However, it isdesirable for both heating element 13 and sensing element 14 to belocated on substrate 11 for efficient heating and space conservation.Recess 12 in substrate 11 assists the heat dissipation or cooling ofheating element 13 and sensor 10.

Coupling lines 15 and 16 electrically couple features 17 and 18,respectively, to sensing element 14. Coupling lines 15 and 16 arecomprised of an electrically conductive material such as, for example, asilicide or a metal. Coupling lines 15 and 16 are formed overlyingelectrically insulating layer 32 and surface 19 of substrate 11 usingtechniques known in the art.

Features 17 and 18 provide electrical contacts for sensing element 14.For instance, assembly wire-bond wires can be coupled to features 17 and18, which can serve as bonding pads. Features 17 and 18 are typicallycomprised of a metal including, but not limited to, gold or copper andare deposited overlying electrically insulating layer 32 and surface 19of substrate 11 using sputtering, electroplating, chemical vapordeposition, or evaporation techniques.

An adhesive 21 overlies coupling lines 15 and 16, overlies electricallyinsulating layer 32, overlies surface 19 of substrate 11, and ispreferably spatially separated from sensing element 14 to avoidcontaminating sensing element 14. Adhesive 21 can be any appropriateorganic or inorganic bonding material such as, for example, a solderpreform, a silk-screened epoxy, or fritted glass. If an electricallyconductive adhesive is used for adhesive 21, an insulating layer (notshown) should electrically isolate coupling lines 15 and 16 fromadhesive 21.

Adhesive 21 couples or adheres electrically insulating layer 32 and amesh, screen, or filter 22 in order to cap or package sensor 10. As aresult, adhesive 21, electrically insulating layer 32, substrate 11, andfilter 22 form a cavity 31. The volume of cavity 31 can be controlled bythe thickness or height of adhesive 21. As illustrated in FIG. 1,sensing element 14 is located inside cavity 31, and features 17 and 18are located outside cavity 31.

Filter 22 is provided over electrically insulating layer 32 and cavity31 to filter, screen out, or prevent undesirable particles or chemicalsfrom entering cavity 31. Filter 22 has a surface 23, an opposite surface24, contact openings 25 and 30, and filtering holes 26, 27, 28, and 29that serve as a filtering mechanism for filter 22 as discussed in moredetail hereinafter.

Filter 22 is preferably spatially separated from sensing element 14 toavoid contaminating or damaging sensing element 14. Filter 22 shouldhave an appropriate thickness such that filter 22 is substantially rigidin order to prevent an elastic deformation of filter 22, in which filter22 can contact and damage sensing element 14.

A wide variety of materials can be used for filter 22 as discussedhereinafter. However, many of the materials used for filter 22 mayoutgas a chemical at the elevated operating temperatures of sensor 10.Preferably, filter 22 is devoid of outgassing a chemical at the elevatedoperating temperatures to ensure an accurate chemical response of sensor10 to the ambient. However, if filter 22 does outgas a chemical, filter22 should not outgas a chemical that is capable of being detected bysensing element 14 in order to ensure accurate environmental monitoringfor sensor 10. Similarly, adhesive 21, electrically insulating layer 32,substrate 11, coupling lines 15 and 16, and features 17 and 18 shouldalso not outgas a chemical that can be sensed by sensing element 14 atthe operating temperatures of sensor 10.

Filter 22 can be comprised of a non-porous material or a porous or gaspermeable material. Examples of potentially suitable non-porousmaterials include, but are not limited to, conventional single crystalsilicon substrates, III-V compound semiconductor substrates, and II-VIcompound semiconductor substrates. Examples of potentially suitableporous or gas permeable materials include, but are not limited to,porous silicon substrates, polymer membranes, porous ceramic, glass,charcoal filters, thermosets, alumina, polyimides, silica, and quartz.

When filter 22 is comprised of a porous or gas permeable material,filter 22 has an additional filtering mechanism that filter 22 does nothave when comprised of a non-porous material. Certain liquids or gasescan penetrate through certain porous or gas permeable materials and canenter cavity 31 without passing through filtering holes 26, 27, 28, or29 of filter 22. Thus, a porous or gas permeable material can extend orenhance the filtering capabilities of filter 22 over that of anon-porous material in order to improve the chemical sensitivity andselectivity of sensor 10.

Each porous or gas permeable material can have a different pore sizethat can be used to filter out different sizes of particles, chemicals,or molecules. The porous or gas permeable materials can be chemicallyactive. As a specific example of a chemically active gas permeablematerial, a layer of a metallophthalocyanine polymer can be used forfilter 22 in order to prevent nitrous oxide from passing into cavity 31.As a specific example of a porous material, a compressed charcoal filtercan be used for filter 22 to filter out and prevent hydrocarbons fromentering cavity 31. Furthermore, a polyimide layer can be used forfilter 22 to filter out and prevent moisture or water vapor fromentering cavity 31.

Referring back to the description of contact openings 25 and 30 withinfilter 22, contact openings 25 and 30 are located over and permit accessto features 17 and 18, respectively. When features 17 and 18 serve asbonding pads, contact openings 25 and 30 each have a dimension ofapproximately 50-1,000 microns to enable assembly wire-bond wires toextend through contact openings 25 and 30 to contact features 17 and 18,respectively. Contact openings 25 and 30 can also expose die singulationareas, identified as lines 36 and 37 in FIG. 1.

Filtering holes 26, 27, 28, and 29 of filter 22 are located over cavity31 and serve as a filtering mechanism for filter 22. While filter 22 canhave a single hole overlying cavity 31, filter 22 preferably has aplurality of holes to permit adequate gas or liquid flow into and out ofcavity 31 while maintaining adequate filtering functionality asdescribed hereinafter. Filtering holes 26, 27, 28, and 29 eachpreferably have a diameter smaller than that of contact openings 25 and30 to prevent unwanted particles from entering cavity 31. Thus, filter22 protects sensing element 14 from physical damage and contaminationduring substrate dicing, other assembly processes, and operation ofsensor 10.

If desired, filtering holes 26, 27, 28, and 29 can each have a diameteron the order of angstroms to microns in order to prevent larger sizedmolecules or chemicals from entering cavity 31 and chemically reactingwith sensing element 14. In this manner, filter 22 is also used as achemical filter to improve the chemical selectivity and sensitivity ofsensor 10. As an example, assume that sensor 10 should only monitorsmall hydrocarbon molecules but that sensing element 14 chemicallyreacts with small hydrocarbon molecules, larger protein molecules, andeven larger deoxyribonucleic acid molecules (DNA). In this example, iffiltering holes 26, 27, 28, and 29 each had a diameter on the order of afew angstroms, small hydrocarbon molecules can pass through filteringholes 26, 27, 28, and 29 to react with sensing element 14 while thelarger protein molecules and the DNA molecules cannot pass throughfiltering holes 26, 27, 28, and 29 and cannot react with sensing element14. Thus, in this example, the chemical selectivity of sensor 10 isimproved.

Filtering holes 26, 27, 28, and 29 and contact openings 25 and 30 aremicromachined into filter 22 prior to coupling together filter 22 andsubstrate 11. Filtering holes 26, 27, 28, and 29 and contact openings 25and 30 can be formed using a variety of different chemical and physicalmethods. For example, a reactive ion etch or a mechanical drillingtechnique can be used to form filtering holes 26, 27, 28, and 29 andcontact openings 25 and 30 in filter 22. As another example, when filter22 is comprised of a non-porous single crystal silicon substrate havinga thickness of approximately 100-500 microns, an anisotropic etchantsimilar to that used for recess 12 in substrate 11 can also be used toetch filtering holes 26, 27, 28, and 29 and contact openings 25 and 30.

Filtering holes 26, 27, 28, and 29 and contact openings 25 and 30 can beetched from surface 23, from surface 24, or from both surfaces 23 and24. As illustrated in FIG. 1, contact openings 25 and 30 and filteringhole 26 are etched from surface 23; hole 27 is etched from surface 24;and holes 28 and 29 are etched from surfaces 23 and 24. When holes areetched from both surfaces 23 and 24, a greater number or a higherdensity of holes can be provided in filter 22 compared to when the holesare only etched from a single surface of filter 22.

Continuing with FIG. 2, a partial cross-sectional view of an alternativeembodiment of sensor 10 in FIG. 1 is portrayed as a sensor 40. Sensor 40of FIG. 2 is similar to sensor 10 of FIG. 1, wherein the same referencenumerals are used in FIGS. 1 and 2 to denote the same elements. In FIG.2, a cavity 44 is formed by using adhesive 21 to couple togetherelectrically insulating layer 32 and a filter 45. Cavity 44 and filter45 are similar in purpose to cavity 31 and filter 22, respectively, ofFIG. 1.

Filter 45 is comprised of a layer 43 overlying a support layer 41.Support layer 41 is similar in composition to filter 22 of FIG. 1.Support layer 41 has a plurality of holes 42, which are covered by layer43 and which are similar in purpose to filtering holes 26, 27, 28, and29 of filter 22 in FIG. 1.

Layer 43 is comprised of a porous or gas permeable material that servesas a selective filter to permit certain chemicals to pass through and torestrict the passage of other chemicals. Examples of porous materialsand gas permeable materials suitable for layer 43 have previously beendescribed herein.

Layer 43 can be sputtered, sprayed, laminated, dispensed, or painted toa thickness of approximately 0.1-30 microns over support layer 41 aftercoupling support layer 41 to electrically insulating layer 32.Alternatively, layer 43 can be provided over support layer 41 beforefilter 45 is attached to electrically insulating layer 32. In thisalternative process, filter 45 can be coupled to electrically insulatinglayer 32 such that electrically insulating layer 32 and substrate 11 arelocated closer to layer 43 than support layer 41, which is aconfiguration that is not shown in FIG. 2. However, filter 45 ispreferably coupled to electrically insulating layer 32 such thatelectrically insulating layer 32 and substrate 11 are located closer tosupport layer 41 than layer 43, as portrayed in FIG. 2, so thatplurality of holes 42 will not become clogged during the operation ofsensor 40.

Referring now to FIG. 3, a partial cross-sectional view of anotheralternative embodiment of sensor 10 in FIG. 1 is depicted as a sensor60. Sensor 60 of FIG. 3 is also similar to sensor 10 of FIG. 1, whereinthe same reference numerals are used in FIGS. 1 and 3 to denote the sameelements. In FIG. 3, adhesive 21 couples together electricallyinsulating layer 32 and a filter 61 to form a cavity 62 therebetween.Cavity 62 and filter 61 are similar in purpose to cavity 31 and filter22, respectively, in FIG. 1.

Filter 61 is comprised of a porous or gas permeable material that has anappropriate thickness to provide substantial rigidity in order toprevent damaging sensing element 14 as previously discussed herein.Unlike filter 22 of FIG. 1, filter 61 of FIG. 3 does not have anyfiltering holes. Filter 61 can be similar in composition to layer 43 ofFIG. 2 and can have a thickness of approximately 50-500 microns.

Sensors 10, 40, and 60 in FIGS. 1, 2, and 3, respectively, have severaladvantages over prior art sensors that are packaged in conventionalmetal T05 or T39 packages. For example, cavities 31, 44, and 62 of FIGS.1, 2, and 3, respectively, have smaller cavity volumes compared to thecavities or enclosed regions of the conventional metal T05 or T39packages. With smaller cavity volumes, sensors 10, 40, and 60 aresmaller in size and more compact than the conventional metal T05 or T39packages, which conserves space in any application. Sensors 10, 40, and60 are at least approximately one hundred times smaller than theconventional metal T05 or T39 packages.

Also, with smaller cavity volumes, cavities 31, 44 and 62 can be filledmore quickly with a critical concentration of a chemical to be sensed bysensing element 14. A smaller cavity volume also permits faster purgingof a critical chemical concentration. Thus, the response and refreshtimes for sensors 10, 40, and 60 are improved over the prior art. Asdiscussed previously, the cavity volumes of cavities 31, 44, and 62 canbe controlled by the thickness or height of adhesive 21. The minimumcavity volume required for cavities 31, 44, and 62 is dependent upon thecomposition and operating temperatures of sensing element 14, theparticular chemical being sensed, and the diffusion rate of an ambientgas or liquid into and out of cavities 31, 44, and 62.

Furthermore, the manufacturing process for sensors 10, 40, and 60 isless time consuming, less expensive, and less labor intensive comparedto the prior art. When substrate 11 and filters 22, 45 or 61 areportions of different semiconductor wafers, the fabrication of sensor 10can be accomplished by using automated semiconductor wafer handlingequipment, which reduces human intervention and improves manufacturingyields. In this manner, the fabrication of sensor 10 is compatible withhigh volume, production environments.

Thus, sensors 10, 40, and 60 can be packaged or assembled using awafer-level batch process, wherein hundreds or thousands of sensors aresimultaneously packaged on a single semiconductor substrate before theindividual sensors are singulated. This wafer-level batch packagingprocess improves throughput and is more cost effective than the manualand tedious prior art process of separately packaging one sensor at atime.

Moreover, the wafer-level packaging protects sensing element 14 frombeing damaged during die singulation because sensing element 14 isenclosed within cavity 31, 44, or 62 prior to the singulation process.Additionally, adhesive 21 and filters 22, 45, and 61 stiffen andstrengthen sensors 10, 40, and 60, respectively, which lowers thepotential for breakage. Accordingly, the manufacturing yields forsensors 10, 40, and 60 are further improved over the prior art.

Therefore, in accordance with the present invention, it is apparentthere has been provided an improved sensor that overcomes thedisadvantages of the prior art. The inefficient, piece-part assembly ofsensors in conventional metal T05 and T39 packages is eliminated, and acost-effective and cycle time reducing method improves the mechanicalstrength and manufacturing yields for fabricating a sensor. The size ofthe packaged sensor is reduced by a factor of greater than approximatelyone hundred compared to conventionally packaged sensors. Furthermore,the performance of a sensor is enhanced by improving chemicalsensitivity, chemical selectivity, and refresh and response times.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be understood by thoseskilled in the art that changes in form and detail may be made withoutdeparting from the spirit and scope of the invention. For instance,humidity and temperature sensors can be included within cavities 31, 44,and 62 to improve the monitoring capabilities of sensors 10, 40, and 60,respectively. Furthermore, the process described herein can be appliedto packaging other types of sensors such as, for example, chemical fieldeffect transistors (CHEMFETs), surface acoustic wave (SAW) devices,capacitive sensors. Accordingly, the disclosure of the present inventionis not intended to be limiting. Instead, the disclosure of the presentinvention is intended to be illustrative of the scope of the invention,which is set forth in the following claims.

What is claimed is:
 1. A method of fabricating a sensor, comprising:providing a substrate; depositing a chemical sensing layer over thesubstrate; patterning the chemical sensing layer; and adhering a filterto the substrate and over the chemical sensing element, the adheringstep occurring after the patterning step.
 2. The method of claim 1wherein the adhering step further comprises keeping the filter separatedfrom the chemical sensing layer.
 3. The method of claim 1 wherein thesubstrate comprises a semiconductor substrate and an electricallyinsulating layer.
 4. The method of claim 1 further comprising etching aplurality of holes into the filter before the adhering step.
 5. Themethod of claim 4 further comprising coupling a layer to the filter tocover the plurality of holes in the filter.
 6. The method of claim 1further comprising: providing the filter with a first surface and asecond surface opposite the first surface; and etching a first pluralityof recesses into the first surface of the filter and a second pluralityof recesses into the second surface of the filter wherein the first andsecond plurality of recesses are aligned to each other and contact eachother at a central portion of the filter.
 7. The method of claim 1further comprising singulating the chemical sensing layer from a waferafter the adhering step.
 8. A method of fabricating a sensor comprising:providing a wafer with a first surface and a second surface; depositingan electrically insulating layer over the first surface of the wafer;etching a recess into the second surface of the wafer; forming a sensingelement on the electrically insulating layer and over the recess;adhering a filter to the electrically insulating layer, the filtercovering the sensing element; and singulating the sensing element fromthe wafer after the adhering step.
 9. The method of claim 8 wherein thedepositing step further comprises forming a heater in the electricallyinsulating layer, over the recess, and under the sensing element,wherein the etching step further comprises exposing a portion of theelectrically insulating layer from the second surface of the wafer, andwherein the forming step further comprises providing a chemicallysensitive film for the sensing element, wherein the adhering stepfurther comprises preventing the filter from physically contacting thesensing element.
 10. The method of claim 8 wherein the adhering stepfurther comprises providing an adhesive layer between the electricallyinsulative layer and the filter to adhere the filter to the electricallyinsulative layer, the adhesive layer spatially separated from thesensing element, and further comprising: forming electrical couplinglines between the adhesive layer and the electrically insulating layerbefore the adhering step, the electrical coupling lines electricallycoupled to the sensing element; and forming bonding pads over theelectrically insulating layer before the adhering step, the bonding padselectrically coupled to the electrical coupling lines and the filterhaving openings to expose the bonding pads.
 11. The method of claim 8wherein the adhering step further comprises selecting the filter fromthe group consisting of a porous material and a gas permeable material.12. The method of claim 8 wherein the adhering step further comprisesetching a plurality of holes into the filter before the adhering step.